1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a backlight unit that enables efficient parallel driving of external electrode fluorescent lamps (EEFLs).
2. Discussion of the Related Art
Cathode ray tubes (CRTs) have been widely used for computer monitors, televisions, measuring machines and information terminals. However, CRT technology fails to meet the current trend of minimizing the weight and size of a display device. Accordingly, display device technology such as liquid crystal display (LCD) devices based on optoelectronic principles, plasma display panel (PDP) devices using a gas discharge, and electroluminescence display (ELD) devices based on the electric field luminous effect have been studied to substitute CRT technology. Among those display devices, the LCD device has been most actively studied.
LCD devices have the advantageous characteristics such as compact size, light weight, and low power consumption. Therefore, LCD devices have been actively used for laptop computers, desktop computers, and large-sized display devices. This trend explains the strong demand for the LCD devices today. Most LCD devices control the light transmittance to display images. To enhance the luminance, it is necessary to form an additional light source such as a backlight unit in an LCD panel. In general, the backlight unit used as the light source of an LCD device has one or more cylindrical fluorescent lamps which may be classified into two types according to the arrangement of the fluorescent lamps: a direct type and an edge type.
For the edge type backlight unit, a lamp unit is provided at one side of a light-guiding plate. The lamp unit includes a lamp for emitting light, a lamp holder inserted into both ends of the lamp to protect the lamp, and a reflective sheet having one side inserted to the light-guiding plate and surrounding the circumference of the lamp to reflect the light emitted from the lamp to the light-guiding plate. In this respect, the edge type backlight unit is generally applied to relatively small sized LCD devices such as laptop computer and the desktop computer.
With the recent trend for the large-sized LCD device having 20-inch screen size or more, the direct type backlight unit has been actively developed. The direct type backlight unit includes a plurality of lamps formed along single line on a lower surface of a light-diffusion sheet, whereby an entire surface of the LCD panel is directly illuminated with the light. The direct type backlight unit, which has greater light efficiency as compared with the edge type backlight unit, is used for the large-sized LCD device because large-sized LCD device requires high luminance. Accordingly, the LCD device of the direct type backlight unit is generally used for the large-sized monitor or television. The large-sized monitor or television may be used for a long time. However, since the direct type backlight unit includes the plurality of lamps, some of the lamps may malfunction.
In contrast, the edge type backlight unit only causes a slight decrease in luminance even though some lamps may fail to function properly. The edge type backlight unit will not significantly affect the luminance on the entire LCD panel. However, the direct type LCD device has a plurality of lamps provided underneath a screen of the LCD panel. Accordingly, if one of the lamps fails due to some reasons, a portion of screen corresponding to the failed lamp becomes darker. Thus, the lamp is required to be exchanged frequently in the direct type LCD device. Accordingly, it is necessary for the direct type LCD device to have a simple structure that allows one to disassemble or to assemble the lamp unit easily.
A related art direct type backlight unit will be described with reference to the accompanying drawings. FIG. 1 is a perspective view illustrating the related art direct type backlight unit, FIG. 2 is a plane view illustrating lamp arrangement of another related art direct type backlight unit, and FIG. 3 is a rear view illustrating the direct type backlight unit of FIG. 2.
As shown in FIG. 1, the related art direct type backlight unit includes a plurality of fluorescent lamps 1, an outer case 3, and light-scattering means 5a, 5b and 5c. The outer case 3 fixes and supports the fluorescent lamps 1. The light-scattering means 5a, 5b and 5c are provided between the fluorescent lamps 1 and an LCD panel (not shown). The light-scattering means 5a, 5b and 5c are used to prevent the silhouette of the fluorescent lamps 1 from being reflected on the display surface of the LCD panel, and to provide a light source with uniform luminance. To improve the light-scattering effect, there is a plurality of diffusion sheets and one diffusion plate is placed between the fluorescent lamps 1 and the LCD panel. In addition, a reflective sheet 7 is provided on an inner surface of the outer case 3 for reflecting the light emitted from the fluorescent lamps 1 to the display part of the LCD panel to improve light efficiency.
The fluorescent lamps 1 are formed of cold cathode fluorescent lamps (CCFLs). Specifically, electrodes are formed at both ends of a tube in each of the fluorescent lamps 1. Both ends of the fluorescent lamps 1 are respectively inserted to holes formed on both sides of the outer case 3. Then, electrodes of the fluorescent lamps 1 are connected with power lead-in lines 9a and 9b to supply voltages to the fluorescent lamps 1. The power lead-in lines 9a and 9b are also connected to a driving circuit by additional connector. Thus, each fluorescent lamp 1 requires an individual connector. Accordingly, when a voltage is applied to the electrodes of the fluorescent lamps 1, the fluorescent lamps 1 emit the light.
The power lead-in line 9 is connected to one electrode of the fluorescent lamp 1 and the power lead-in line 9a is connected to the other electrode of the fluorescent lamp 1. Both the power lead-in lines 9 and 9a are connected to a single connector. Therefore, since each one of the fluorescent lamps 1 requires the additional connector, the length of one of the power lead-in lines connected to the single connector may become longer. Thus, connection among the connectors and the power lead-in lines becomes complicated and the voltage may not be supplied to the fluorescent lamps.
Next, another related art backlight unit will be described. As shown in FIG. 2, a plurality of ‘U’ shaped cold cathode fluorescent lamps (CCFLs) 21 includes first and second electrodes 22a and 22b respectively formed inside an appropriate tube ends of the CCFLs 21. The CCFLs 21 are arranged on a lower structure 20 at constant intervals. The first and second power supply lines 23a and 23b are connected with the first and second electrodes 22a and 22b, respectively, to apply voltages to two electrodes. FIG. 3 shows an inverter 24 arranged on a rear surface of the lower structure 20 and includes a plurality of connectors C1, C2, . . . , Cn−3, Cn−2, Cn−1, Cn connected with the power supply lines of the CCFLs 21. A fluorescent layer is coated on inner surface of the CCFLs 21 and the CCFLs 21 are filled with Ar and Hg gases.
In the related art backlight unit described with respect to FIGS. 2 and 3, the connectors C1, C2, . . . , Cn of the inverter 24 are respectively connected to the first and second power supply lines 23a and 23b of each CCFL 21. If the lamps are arranged in a ‘U’ shape as shown in FIG. 2, the first and second power supply lines connected to a single connector have the same length, thus, the problem of the lengthy power lead-in lines can be solved. However, since the connectors are respectively connected to the first and second power supply lines (power lead-in lines), signals are output from the respective connectors simultaneously to drive the CCFLs simultaneously, therefore, signals output from the inverter increase. Accordingly, driving method for the CCFLs is still complicated.